The present invention relates to a method for shaping a tubular member. More particularly, the present invention is concerned with a method for shaping a tubular member, such as a cylinder for a gas spring used in a vehicle, to thereby form an outwardly projecting portion of the tubular member or form an inwardly projecting portion of the tubular member.
Conventionally, in order to form an outwardly projecting portion of a tubular member, a die having a recess is brought into contact with an outer surface of the tubular member, and the tubular member is pressed by a roller from the inside so that the wall of the tubular member is pushed into the recess of the die, to thereby deform the tubular member. In this case, a problem arises, such that when the tubular member is relatively elongated, it is difficult to apply, by the roller, a force sufficiently large for deforming the tubular member.
On the other hand, in order to form an inwardly projecting portion of a tubular member, a die having a recess is brought into contact with an inner surface of the tubular member, and the tubular member is pressed by a punch from the outside so that the wall of the tubular member is pushed into the recess of the die, to thereby deform the tubular member. In this case, in order to remove the die from the tubular member after deformation, it is necessary to use a die comprising a plurality of components, and it is also necessary to use a mechanism for enabling this plurality of components of the die to be successively removed from the tubular member. This leads to a problem, such as an undesirably high operation cost for shaping the tubular member.